1. Field of the Invention
The present invention relates to devices capable of displaying images and processing audio messages.
2. Description of the Prior Art
Liquid crystal displays (LCDs) are used in a variety of small-sized devices such as cellular phones and personal digital assistant (PDA) devices. These small-sized devices usually require an extremely small pitch of connections of a driver large-scale integrated circuit (LSI). Hence, an LCD driving device is generally used, which integrally incorporates a driver by using polysilicon thin film transistors (TFTs). A TFT LCD panel thus comprises a matrix of pixels, thousands or millions of which together create an image on the display, and corresponding TFTs acting as switches individually to turn each pixel on or off. Since many of these small-sized devices are portable, battery-operated devices, low power consumption is an important display attribute. For devices such as mobile phones and PDAs with TFT LCD panels, that can display images and receive audio messages, screen flicker and audible noise are also important concerns.
Reference is made to FIG. 1, which schematically depicts a portion of a prior art TFT LCD panel 10. A pixel element 12 of the TFT LCD panel 10 includes a storage capacitor Cs and a liquid crystal capacitor CLC for data storage. A TFT 14, acting as a switch for turning the pixel element 12 on or off, is controlled by a gate voltage typically applied with between −5V to 20V by a power line GATE. The video source provided by a power line SOURCE, typically ranging from 0V and 10V, provides the intensity information that appears across the pixel element 12. A bottom of the pixel element 12 is commonly connected to a back plane of the panel. The voltage at this node is known as Vcom.
In a TFT LCD device, the magnitude of the applied source voltage determines the intensity of light emitted by the pixel. Assuming the Vcom voltage is at ground, the voltage across the pixel varies from 0 V to 10 V. Assuming an average of 5 volts, there is substantial DC voltage across each pixel. This DC voltage causes charge storage, or memory. In the long term, this DC voltage ages and degrades the pixels by electroplating ion impurities onto one of the electrodes of the pixel. This contributes to image retention, commonly known as a sticking image. For preventing orientation films and liquid crystals from deteriorating due to electrochemical reactions, as well as preventing sticking or persistence of image, the polarity of the pixel voltage is reversed on alternate intervals, known as the frame rate of the TFT LCD panel. The typical frame rate used for the TFT LCD panel is about 60 Hz.
Flicker is an artifact that makes an image appear to flash rather than retain steady brightness. The minimum frequency at which a modulated source is perceived as steady is known as the critical flicker frequency (CFF). Flicker is perceived when the frequency of modulated light falling on the retina of the human eye is below the CFF. Since the field display rate a measurement of how quickly a display device can produce unique consecutive images called frames), as well as the effective frame rate (the average field display rate of a display device), of most displays are below most people's CFF, flicker is often noticeable and detracts from the image quality. By operating a TFT LCD panel with a higher frame rate, the undesirable flicker effect can be significantly reduced.
Also, the noise level is related to Vcom driving signal frequency. For a TFT LCD panel with QVGA resolution (240×RGB×320), the Vcom frequency can be represented as follows:Vcom frequency≅frame rate (usually 60 Hz)×160=9.6 kHz
Though this noise level is very low, it is still noticeable when the TFT LCD device is put close to a human ear. This can happen when a user talks with a cellular phone or a PDA after receiving or making a phone call. The audible noise within close vicinity to the TFT LCD device is undesirable since the noise can affect the quality of communication.
In the prior art, the frame rate for driving a TFT LCD panel is fixed. To reduce the undesirable flicker effect and the noise level, a conventional method is usually applied by operating the TFT LCD panel with a higher frame rate. However, the power consumption of the TFT LCD panel is proportional to ½CV2F, where F is operating frequency, V is operating voltage and C is panel capacitance. Therefore, while higher frame rate reduces flicker and noise significantly, it also increases the power consumption of the TFT LCD device. Thus, the prior art method reduces flicker and noise by operating the TFT LCD device at a fixed higher frame rate, but it also increases the overall power consumption.